The convergence or the joining together of time division multiplexing-based networks—also known as TDM networks—and packet-based networks has resulted in new switching requirements for in terms of converting signaling and useful data.
In the course of tuning time division multiplexing-based and packet-based networks, the term interworking is frequently used here, adjustments are required not only at the subscriber end but also at the exchange node end.
A TDM-based exchange or a TDM-based exchange node instigates the data link control and the scheduling or through switching of the bearer channels. The range of tasks of an exchange transmitting data in the packet-switched network—hereinafter called packet related exchange—includes, however, in addition to the usual data link control, the control of the associated bearer channels run outside the packet-based exchange, said control being implemented by means of external devices (e.g. Gateways, Resource Servers, . . . ), which provide suitable interfaces for useful data streams (e.g. using the RTP (Real Time Protocol)) and control (e.g. using one of the following protocols: MGCP (Media Gateway Control Protocol), H.248, H.323).
For the terminal side, in addition to the classic analogue and ISDN terminals and private branches, there are also terminals suitable for the packet-switched network, which terminals allow broadband data access but that in addition are capable of supporting the basic performance features known from public telephone networks (e.g. using the H.323 protocol or the SIP (Session Initiation Protocol) protocol). Extension circuits or cable networks operated using the appropriate xDSL technology (DSL: Digital Subscriber Line) are often used as a broadband access medium or access network to the subscriber.
The interface between access network and transmission network, e.g. packet-based IP (Internet Protocol) network, is usually formed using subscriber end adaptation devices. The term “peripheral adapters” is often used for subscriber end adaptation devices. Examples of subscriber end adaptation devices are IAD (Integrated Access Device) for scheduling an xDSL line and MTA (Multimedia Terminal Adapter) on or in the Cable Modem, which terminates the access network on the transmission network side. Thus using subscriber end adaptation devices and access networks, it is possible to connect broadband terminal devices (e.g. PC with Internet access, tv set, videophone) as well as traditional subscriber terminal units, e.g. analogue telephone, ISDN telephone, and also analogue and ISDN private branch exchange.
Packet-oriented exchanges should, as far as possible, provide all the performance features known in line bound connection technology for traditional terminal units, i.e. telephones and private branch exchanges so that the inclusion of packet-oriented networks does not lead to the range of service characteristics being restricted. In addition, if hardware and software resources are not available or terminal units are not accessible, this must be detected and if necessary the operator alerted to such fact. As a rule, the packet-oriented exchange node does not know the structures of the intermediary access networks. For example, situations where components or network parts fail and which affect the signaling path between the packet-oriented-exchange node and subscriber terminal device, are only indirectly apparent to the exchange node, i.e. through detection of the non accessibility of subscribers and private branches.
Traditional analogue and ISDN subscribers accessible via a packet-switched network are identified in a special way for administration in the packet-based exchange node to differentiate them from purely packet-based subscribers (SIP, H.323) and traditional, analogue and ISDN subscribers connected by telephone line (e.g. connections via the interfaces V5.1 and V5.2). The peripheral, subscriber end adaptation devices possess special functions (e.g. telephony client in IADs and MTAs), which make it possible to transmit the signaling between terminal unit and packet-based exchange via intermediary access networks and packet-based transmission networks or wide area networks. The packet-based exchange systems and clients in the peripheral adaptation devices (IAD, MTA) can, in addition, be correspondingly configured by the network administration. The exchange system differentiates analogue and ISDN interfaces when it provides functions in the subscriber end adaptation devices for transmitting signaling. For the packet-based exchange system a further differentiating feature is the allocation of performance features of a main station or of a private branch connection. From the subscriber's view, then either an analogue or an ISDN terminal unit operated as a main station is connected to the peripheral adapter (IAD, MTA), or similarly an analogue or ISDN private branch exchange (also called PBX) is connected.
With respect to ISDN interfaces, e.g. on the peripheral adaptation device, two different connections or interfaces are differentiated:                In the case of the basic connection, also referred to as BRA (Basic Rate Access), there are one or two bearer channels available. Frequently there are two bearer channels (B-channel) each of 64 kbit/s (56 kbit/s in USA) and a signaling channel (D-channel) of 16 kbit/s.        The second type of connection, also referred to as PRA (Primary Rate Access), is the primary multiplex connection, comprising, as a rule, 30 B-channels (because of the PCM30 system), a synchronization channel and a signaling channel (D-channel). According to standardization, up to 4 PCM30 systems can also be put together with a single D-channel to form a PRA.        
As opposed to the case with a packet-based exchange, the status (layer 1, layer 2, blocking status) of the ISDN connection (BRA, PRA) is known to a traditional local exchange of a TDM network because of physical line scheduling or because of the interface protocols for subscriber line concentrators and access networks.
It must also be possible for packet-switched networks to use peripheral adapters to connect small ISDN private branches via a BRA or a number of BRAs; it must be possible to link large ISDN private branches via one or a number of PRAs, whereby it must be possible to support a mixture of BRAs and PRAs with respect to one given ISDN private branch. Hereby, BRA and PRA are possibly connected via one standard adapter or via specific peripheral adapters and physically timed. The ISDN signaling can then be exchanged packet-based between peripheral adapter and packet-oriented exchange (e.g. using ISDN User Adaptation IUA and SCTP). This usually occurs by means of the DSS1 (Digital Signaling System No. 1) protocol, details of which protocol are set out in the Standard ITU-T Q.931 (ISDN user-network interface layer 3 specification for basic call control).